Non-equilibrium molecular dynamics (NEMD) method was used to simulate the thermal conductivity of monocrystal germanium thin films on the out-of plane. In the simulation, the thickness of the films ranged from 2.8288 to 11.315 nm and the average temperature was 400 K. The results of calculations demonstrate that the thermal conductivity of the monocrystal germanium thin film increases nearly linearly with increasing of the film thickness, which is remarkably lower than the corresponding experimental data of bulk germanium. For a given film thickness, the thermal conductivity of the nanometer thin film varies slightly with increasing of the temperature, and its size effect is significant comparing with the thermal conductivity of bulks.